Most high voltage bushings have one dielectric tube external to a building or container. Feed-through bushings that exit from a building often have a second dielectric tube internal to the building. The tubes are usually joined together by a coupling where the bushing passes through the building. Often the tubes are sealed and pressurized with air, nitrogen (N2) or sulfur hexaflouride (SF6) to increase the withstand voltage between a high voltage center conductor mounted in the bushing and the ground potential at the point where they penetrate the building. The dielectric tubes are most commonly made out of fiberglass covered with silicone rubber materials. Recently, these bushings have included internal field shapers for moderating the electric field along the bushing. U.S. Pat. No. 6,951,987, entitled, “High Voltage Bushing” describes an exemplary embodiment of a bushing utilizing an internal field shaper.
High-voltage feed-through bushings typically weigh thousands of pounds and may be up to twenty feet in length. Typically, these bushings are assembled vertically. Vertical assembly of these high-voltage feed-through bushings presents a number of problems. First, assembling the bushing outdoors could result in physical dangers to personnel and damage to equipment as a result of inclement wind and weather. Second, assembly outdoors presents a greater risk of contamination within the interior of the bushing. Third, assembling the bushing indoors requires very tall facilities. Fourth, expensive and heavy-duty over-head hoists or cranes are required for vertical assembly of bushings. Fifth, alignment with vertical assembly can be difficult and hazardous because of the need for an individual to be present at the height of assembly.
The process of assembling a high-voltage feed-through bushing should be relatively safe and inexpensive, should minimize the chance of contamination of the interior of the bushing, and should not require specialized facilities.